Sensing a current which is flowing along the main conduction path of a transistor, i.e., the collector-emitter current for BJT transistors and the source-drain current for MOSFET transistors, is a fairly common practice, especially with power transistors. Sensing a current can be useful to protect the transistor against overloads or, in certain applications, to control the current being delivered to the load by the transistor.
The most common method for effecting this sensing is to connect, in series with the main conduction path of the transistor, a resistor having an exactly known, very low and stable value, and then measure the voltage drop across the resistor. However, the voltage drop introduced by the resistor is somewhat disadvantageous for certain applications. In addition, the resistor dissipates a certain amount of electric power which is undesirable.
To overcome such drawbacks, another method has long been in use for semiconductor transistors of the integrated type, which consists of fabricating two transistors on the same chip, namely, a conduction one, indicated as PT in FIG. 1, and a sense one, indicated as PS in FIG. 1, which transistors differ from each other by their conduction areas. If their control voltages, VGS in the instance of the MOSFET transistors in FIG. 1, are made identical, then the ratio of the currents flowing through them will be equal to the ratio of their conduction areas. FIG. 1 is described more fully below.
As is well known, power transistors of the integrated type are often formed by a plurality of identical elements, usually known as "cells. " FIG. 2B shows a cross-section of an exemplary cell for MOSFET transistors, and FIG. 3 is a schematic top view of a power transistor PW comprising a plurality of such cells located within a region RG of the chip. FIG. 3 shows, as an example, an array of 12 columns and 9 rows.
The cell of FIG. 2B is formed within an epitaxial layer EPI of the N-type, overlying a substrate SUB of the P type, which layer constitutes the drain terminal DT. The cell is formed by a bulk well BD of the P type wherein two source wells SD of the N+ type are provided. The wells RD and SD are surface contacted together by a metal structure forming the source terminal ST. Located at the surface included between the edges of the wells SD and the edge of the well BD are two polysilicon structures which form the control terminal GT and are isolated from the surface by an insulating material. When looked from the top, the cell appears as a closed, e.g., circular, area corresponding to the well BD and wherein is a girdle, e.g., a circular one, corresponding to the wells SD.
The cells forming the power transistor PW are divided into two sections: (i) the conduction elements CE which are connected together in parallel and form the conduction transistor PT, i.e., the conduction section of the power transistor PW, and (ii) the sense elements SE which are connected together in parallel and form the sense transistor PS, i.e., the sense section of the power transistor PW.
As shown in FIG. 3, a set of (eight in the example) elements next to each other are usually selected for the sense elements SE. In this way, the necessary connections become easier to make. In the example of FIG. 3, the ratio of the conduction areas is 8/100. In practical applications, this ratio is generally much lower, e.g., 8/8000=1/1000. FIGS. 2B, 3, and also 2A are described more fully below.
By this method, definitely better sensing results are obtained than those to be obtained with the resistor, and with none of its disadvantages. By first approximation, no sensing errors occur. Of course, the area occupied by the transistor on the chip is slightly larger. However, it has been found by second approximation analysis that the current sensed by the sense transistor PS is not exactly proportional to that flowing through the conduction transistor PT. This discrepancy has been attributed to the fact that, in electric operation, the region RG occupied by the power transistor has a temperature which varies according to position. For example, the middle zone may be at 70.degree. C. and, at the same time, the peripheral zones be at 50.degree. C. Furthermore, each of the conduction and sense elements is affected by its instant temperature, so that the current through them will be different for the same control voltage.
It is the object of this invention to provide a method of sensing a current of a transistor which is more accurate than conventional methods and to provide transistors for implementation of the method.
Further advantageous aspects of this invention are set forth below.